Interpretive Summary: Winter cover crops are increasingly common on organic and conventional vegetable farms on the central coast of California between periods of intensive, irrigated vegetable production. A 2-yr study was conducted in Salinas, California, to quantify (1) cover crop and weed biomass production during cover cropping, (2) early season canopy development of cover crops, (3) weed seed production by burning nettle during cover cropping, and (4) weed emergence following cover crop incorporation. Cover crops included oats, rye, a mustard mix, and a legume/oats mix, that were planted in October and soil-incorporated in February. Measurements included weed and cover crop densities, early-season cover crop canopy development, above ground weed and cover crop biomass production, seed production by the burning nettle, and post-incorporation weed emergence. Mustard and rye produced more early-season biomass than oats and the legume/oats mix, but by the end of the season there were no differences in above ground biomass production by the cover crops. Suppression of weed biomass, and seed production of burning nettle was greatest in mustard and rye, and least in oats and the legume/oats mix. Weed suppression by the cover crops was consistent with their early season canopy development and was highly correlated with planting density. Weed emergence following cover crop incorporation was in order of legume/oats mix > oats > mustard in yr 1, but was not different between cover crops in yr 2. This study provided some of the first information on cover crop effects on weed management in irrigated and tilled vegetable production systems in the central coast of California. The data suggest that the legume/oats mix could exacerbate weed problems in subsequent vegetable crops.

Technical Abstract:
Winter cover crops are increasingly common on organic and conventional vegetable farms on the central coast of California between periods of intensive, irrigated vegetable production. A 2-yr study was conducted in Salinas, California, to quantify (1) cover crop and weed biomass production during cover cropping, (2) early season canopy development of cover crops, (3) weed seed production by burning nettle during cover cropping, and (4) weed emergence following cover crop incorporation. Cover crops included oats, rye, a mustard mix, and a legume/oats mix, that were planted in October and soil-incorporated in February. Measurements included weed and cover crop densities, early-season cover crop canopy development, above ground weed and cover crop biomass production, seed production by the burning nettle, and post-incorporation weed emergence. Mustard and rye produced more early-season biomass than oats and the legume/oats mix. By the end of the season there were no differences in above ground biomass production by the cover crops. Suppression of weed biomass, and seed production of burning nettle was greatest in mustard and rye, and least in oats and the legume/oats mix. The weed suppressive ability of the cover crops was consistent with their early season canopy development and was highly correlated with planting density. Weed emergence following cover crop incorporation was in order of legume/oats mix > oats > mustard in yr 1, but was not different between cover crops in yr 2. This study provided some of the first information on cover crop effects on weed management in irrigated and tilled vegetable production systems in the central coast of California. The data suggest that the legume/oats mix could exacerbate weed problems in subsequent vegetable crops.